[ValhallaGH]

That's a tough request. Both systems are notable for only needing the main book, with all other purchases being supplements to enhance a specific style or genre of game play.

M&M: Instant Super Heroes - at least three variations on each of dozens of heroic archetypes. All PL 10, all easily adjusted to be custom heroes or villains.
Freedom City setting - dozens of organizations, villains, heroes, and a very kick-ass setting all ready to go. Perfect for theft or actually running campaigns in (I've used both methods with great success).

True20: The Companion is included in the Revised edition, which is why we don't bother recommending it if you've got that book (and heartily endorse it if you don't have the Revised edition).
Which two others you'll want. ... Tough call. Most of the great books are settings, which come with lots of neat flavor and crunch designed to promote that flavor. I'm pretty blah on the rest, though the Adept's handbook is great for powers-oriented campaigns.